I. Field of the Invention
The present invention relates generally to satellite communication systems, and more particularly, to a method and apparatus for paging a user terminal located within a strongest signal coverage area or xe2x80x9csweet spotxe2x80x9d of a satellite.
II. Related Art
Conventional satellite-based communication systems include gateways and one or more satellites to relay communication signals between the gateways and one or more user terminals. A gateway is an earth station having an antenna for transmitting signals to, and receiving signals from, communication satellites. A gateway provides communication links, using satellites, for connecting a user terminal to other user terminals or users of other communication systems, such as a public switched telephone network. A satellite is an orbiting receiver and repeater used to relay information. A user terminal is a wireless communication device such as, but not limited to, a wireless telephone, a data transceiver, and a paging receiver. A user terminal can be fixed, portable, or mobile, such as a mobile telephone.
A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the xe2x80x9cfootprintxe2x80x9d of the satellite. The footprint of a satellite is the geographic region on the surface of the Earth within the range of signals of the satellite. The footprint is usually geographically divided into xe2x80x9cbeams,xe2x80x9d through the use of beam-forming antennas. Each beam covers a particular geographic region within the footprint. Beams may be directed so that more than one beam from the same satellite covers the same specific geographic region.
Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,xe2x80x9d and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, entitled xe2x80x9cMethod and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy,xe2x80x9d both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.
In satellite communication systems employing CDMA, separate communication links are used to transmit communication signals, such as data or traffic, to and from a gateway. The term xe2x80x9cforward communication linkxe2x80x9d refers to communication signals originating at the gateway and transmitted to a user terminal. The term xe2x80x9creverse communication linkxe2x80x9d refers to communication signals originating at a user terminal and transmitted to the gateway.
On the forward link, information is transmitted from a gateway to a user terminal over one or more beams. These beams often comprise a number of so-called subbeams (also referred to as frequency division multiple access (FDMA) channels, or CDMA channels) covering a common geographic area, each occupying a different frequency band. More specifically, in a conventional spread-spectrum communication system, one or more preselected pseudorandom noise (PN) code sequences are used to modulate or xe2x80x9cspreadxe2x80x9d user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the data signal. On the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different gateways or over different beams, as well as between multipath signals. These codes are often shared by all communication signals within a given subbeam.
In a conventional CDMA spread-spectrum communication system, xe2x80x9cchannelizingxe2x80x9d codes are used to discriminate between different user terminals within a satellite sub-beam on a forward link. The channelizing codes form orthogonal channels in a subbeam over which communication signals are transferred. That is, each user terminal has its own orthogonal channel provided on the forward link by using a unique channelizing orthogonal code. Walsh functions are generally used to implement the channelizing codes, also known as Walsh codes or Walsh sequences, and create what are known as Walsh channels. A typical orthogonal code length is 64 code chips for terrestrial systems and 128 code chips for satellite systems.
A majority of the Walsh channels are traffic channels that provide messaging between a user terminal and a gateway. The remaining Walsh channels often include pilot, sync, and paging channels. Signals sent over the traffic channels are generally meant to be received by only one user terminal, although messages can also be broadcast to multiple users. In contrast, paging, sync, and pilot channels may be monitored by multiple user terminals.
When a user terminal is not involved in a communications session (that is, the user terminal is not receiving or transmitting traffic signals), the gateway can convey information to that particular user terminal using a signal known as a paging signal (also referred to herein as a page). Paging signals are often sent by the gateway to establish a communication link, to tell a user terminal that a call is coming in, to reply to a user terminal trying to access the system, and for registration of the user terminal. For example, when a call or request for a communications link has been placed to a particular user terminal, the gateway alerts the user terminal by means of a paging signal. Additionally, if the gateway is sending a short message to a user terminal, such as a request for a location update of the user terminal, the gateway can send such a request by means of a paging signal. Paging signals are also used to distribute channel assignments and system overhead information. Paging signals are usually transmitted over paging channels, which are briefly discussed above. Each paging signal includes an identity number so that the user terminals listening to the paging channel know if the paging signal is addressed to them. If a paging signal is meant for multiple user terminals, the paging signal includes an identity number that corresponds to the multiple user terminals.
A user terminal can respond to a paging signal by sending an access signal or access probe over the reverse link (that is, the communications link originating at the user terminal and terminating at the gateway). The access signal is also used to register with a gateway, to originate a call, or to acknowledge a paging request by a gateway. The access signal is usually transmitted over channels specifically designated as access channels, which are briefly discussed below. The reverse link also includes traffic channels for providing messaging between a user terminal and a gateway.
If a user terminal is merely sending a location update in response to a location update request that is received from a gateway over a paging channel, the user terminal may send location update information as an access probe over an access channel. By using paging channels and access channels to convey short messages (such as location update requests and location update information), forward and reverse traffic channels are reserved for longer communications such as voice calls.
When a gateway sends a paging signal to a user terminal, the gateway usually does not know the location of the user terminal. Therefore, in contemporary satellite communications systems, the gateway usually sends a paging signal over many paging channels. At worst, the gateway sends the paging signal over every paging channel that is supported by the gateway serving the particular user terminal. This sending of a paging signal over many paging channels is often referred to as flood paging. Flood paging, though inefficient and wasteful, is relatively inexpensive when used to set up voice calls. This is because the resources used to flood page are relatively small compared to the resources used for a typical two or three minute voice call. More specifically, the total capacity and power used to flood page is relatively small compared to the total power and capacity used to support the voice call. Thus, flood paging, though not efficient, has proven useful in voice systems. However, flood paging may become unacceptable when used for setting up voice calls, if, for example, the number of call set up requests increases to the point where paging channel capacity becomes a scarce resource.
The inefficiencies of flood paging are not acceptable in many other types of messaging systems, such as in a position determination system where the response to a paging message may be a relatively short acknowledgment message and/or a location update message. This is because the resources used to flood page are quite large as compared to the information sent in response to the flood page. More specifically, the total power and capacity used to flood page is relatively large compared to the total power and capacity used to support the response to the flood page (for example, an acknowledgment or location update message).
An example of an industry in which position determination is particularly useful is the commercial trucking industry. In the commercial trucking industry an efficient and accurate method of vehicle position determination is in demand. With ready access to vehicle location information, a trucking company home base obtains several advantages. For example, a trucking company can keep the customer informed of location, route and estimated time of arrival of payloads. The trucking company can also use vehicle location information together with empirical data on the effectiveness of routing, thereby determining the most economically efficient routing paths and procedures.
In order to minimize the power and capacity used to track the location of a truck, a location update request can be sent to a user terminal (often referred to as a Mobile Communications Terminal or MCT in the trucking industry) within the truck periodically (for example, once every hour). To further save resources, the collection of location updates should be accomplished without utilizing traffic channels. To accomplish this, a location update request message can be sent as a paging signal over a paging channel. To further minimize the power and capacity used, the number of paging channels used to transmit the paging signal should be minimized for the reasons discussed above. To even further minimize the power used, the paging signal should only be sent when the user terminal (being paged) is within the xe2x80x9csweet spotxe2x80x9d of the satellite(s) (that is used to send the paging signal). The xe2x80x9csweet spotxe2x80x9d of the a satellite is an area or region of coverage by the satellite beams where the gain of signals sent from that satellite is high, referred to as peaked up. That is, where the strength of the signals is relatively high. This is an area that is substantially better for signal propagation for the user terminal.
Thus, as discussed above, there is a need for an apparatus and method for reducing the number of paging channels used to page a user terminal. Additionally, there is a need to minimize the amount of power, per channel, used to page a user terminal. Even though the initial need for the reduction of flood paging was inspired by the reduction of flood paging in a position determination system, the system and method of the present invention is useful in many types of satellite communication systems that uses channels (identical to or similar to paging channels) for conveying information to a user terminal that is not involved in a communications session.
The present invention is most useful when used for an application where a user terminal does not need to be immediately paged. That is, the present invention is most useful for an application where a gateway can wait, until certain conditions are satisfied, before it sends a page to a user terminal. For example, the present invention is useful in a system where paging channels are used for requesting location updates.
The present invention is directed toward a method and apparatus for paging a user terminal (UT) using a satellite communications system having a gateway and plurality of satellites, wherein each satellite produces a plurality of beams and each beam includes a plurality of channels. The method of the present invention includes the step of recalling a location of the UT, wherein the recalled location corresponds to a location of the UT at a time t1. In one embodiment, this is accomplished by using a look-up table, database, or memory elements in which location information for user terminals at different points in time is stored. The method also includes the step of determining an area, based on the recalled location, within which the UT is assumed to be located at a time t2, where time t2 is later in time than time t1. The next step is to determine a time t3, where t3 is equal to or later in time than time t2, when the UT is located within a predetermined preferential portion of the satellite footprint, also referred to as the xe2x80x9csweet spotxe2x80x9d of the satellite, which is defined as a region where the gain of signals transmitted by the satellite is highest. The xe2x80x9csweet spotxe2x80x9d of a satellite can be specified from the perspective of a satellite or of a point on the Earth, such as the recalled UT location. The preferential location can be selected using elevation angles for the satellite relative to the UT of interest. For example, a user terminal is considered to be within the xe2x80x9csweet spotxe2x80x9d of a satellite when the following two criteria are satisfied. First, a satellite of the plurality of satellites has an elevation angle between two pre-selected angles xcex81 and xcex82 from the perspective of the user terminal. Second, all locations within the area determined above are located within a footprint of the satellite (that has an elevation angle between xcex81 and xcex82). The elevation angles xcex81 and xcex82 can be determined from the perspective of the recalled location. Alternatively, the elevation angles can be determined from the perspective of all locations within the area. This ensures that the UT is in the xe2x80x9csweet spotxe2x80x9d of the satellite (where the gain of signals sent from the satellite are highest) when the UT is paged. A page is then sent from the gateway to the UT on a channel of a beam of the satellite at time t3. The page can be sent on a channel of every beam of a plurality of beams that make up the footprint of the satellite at time t3. Alternatively, the page can be sent on a channel of selected beams of the footprint. By waiting until the UT is within the xe2x80x9csweet spotxe2x80x9d to page the UT, less power can be used by the satellite. In addition, this type of operation allows the UT to use an antenna that has a radiation pattern that exhibits a substantial or higher gain only at higher elevation angles or when the UT is within the xe2x80x9csweet spotxe2x80x9d. This allows the use of satellite power to be reduced further.